Yao N. Focused Ion Beam Systems: Basics and Applications

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effectively accomplishing the milling. The graph shows the corresponding SE

signal. As suggested in the graph, the FIB scanning is usually slower than the

SEM although this is not a strict necessity. The signal that forms the image is

the total SE signal, or the sum of both graphs in Figure 5.8. Two effects can

be observed in ‘‘live’’ SE images: ﬁrst, an offset in brightness compared to

SEM-only images (because of the DC part of the FIB generated SE signal);

and second, an interference pattern, corresponding to the peaks in the FIB

generated SE signal.

Although physically impossible to eliminate, a couple of things can be done

to suppress these usually unwanted and overlapping effects. The main

parameter involved is the ratio between the primary currents: if the SEM

current is sufﬁciently high with respect to the FIB current, the interference

and offset effects will be hardly noticeable. In practice, a factor of ﬁve or

higher in current is desirable. Also using a fast scan for the SE SEM imaging,

in combination with frame averaging, will improve the image quality. In fact,

such a scanning strategy will ‘‘smear out’’ the interference and transform it

into an additional brightness offset (which can be eliminated in a straight-

forward way by decreasing the brightness setting of the SEM image). Also

the scan speed ratio as well as the FIB scan speed itself have an inﬂuence;

however, it is impossible to go into details of their much more complex

behavior within the scope of this text.

A completely different, but very effective approach is to use backscattered

electron imaging for SPI mode. Since the FIB obviously does not create

backscattered electrons, the offset and interference on the BSE signal will be

minimal. A possible disadvantage is that a suitable detector has to be present,

and the signal level for imaging is intrinsically lower. Also the need to work

at relatively high SEM acceleration voltages may be disadvantageous on

some materials, especially considering the better charge neutralization

properties of lower-energy electrons.

Finally it should be mentioned that although aesthetically not desirable,

the artefacts in SE images can be very informative to the operator, since they

offer instant and direct information about the interaction of the ion beam

with the sample.

5.3.2 Applications of simultaneous patterning and imaging

As mentioned before, simultaneous patterning and imaging (SPI) is strictly

limited to DualBeams. In everyday DualBeam use, SPI mode is used quite

often, mainly to monitor the progress of a milling or deposition step.

It should be noted that when SPI mode is used to monitor FIB deposition

Focused ion beam systems138

processes, smaller amounts of material may be deposited in the raster ﬁeld as

a result of the scanning electron beam. We believe SPI is at its best when used

for process monitoring and end-point determination.

When milling a (single) cross section, it is often important to stop the

polishing step at an exact position in the sample (e.g. through the center

of a unique defect, through the void in a semiconductor plug as shown in

Figure 5.9). This end-point determination can be realized very elegantly using

SPI, since the operator gets continuous and instant feedback about the

polishing process (from an applications point of view, this is the main dif-

ference from serial sectioning). As soon as the feature of interest appears in

the live SEM image, the FIB polishing process is stopped by the operator.

The last step in FIB TEM preparation is to mill a portion of the specimen

thin enough to be electron transparent (e.g., 100 nm or less) [7]. This last

step in the process is conveniently monitored by (i) measuring the specimen

thickness via top-down FIB imaging in conjunction with (ii) monitoring the

remaining thickness of the protecting layer (usually Pt) on the top of

the specimen. When this protective layer starts to recede and approach the

specimen surface, the FIB thinning process should be stopped. SPI mode

proves to be an efﬁcient means to monitor this process (i) without imaging

unnecessarily with the FIB and (ii) to precisely stop the FIB milling at the

appropriate moment. Furthermore, regions that are FIB milled to electron

500 nm

Figure 5.9 Example of an SE SEM image of an FIB milled cross section

through a plug in a semiconductor device. SPI mode was used to determine

the end point of the polishing process, resulting in a cross section through the

center of the plug where the void is located.

Characterization methods using FIB/SEM DualBeam instrumentation 139

transparency for TEM will also exhibit changing contrast as observed with

the SEM. In particular, as the specimen thins, secondary electrons will not

only be emitted from the front surface, but also from the back surface,

resulting in higher SEM contrast levels at the thinnest portion of the speci-

men. SPI mode can be utilized to monitor this contrast change for precise

end-pointing of TEM specimen preparation. Of course, this effect is quite

complex and depends on the energy of the primary electrons, the target

specimen and the relative foil thickness.

The nature of a multi-step process is often advantageous for using SPI

mode. In addition, as the process nears its ﬁnal steps, a more stringent

requirement for precision using a lower ion beam current usually goes hand

in hand. The former simply justiﬁes the use of SPI, whereas the latter makes

it easier to use SPI mode (e.g., lower FIB current means less interference).

5.4 Scanning transmission electron microscopy (STEM)

characterization in the DualBeam

Even though, strictly speaking, STEM is realized using the electron beam

only (using an SEM in this case), we consider it relevant enough to discuss it

here since the preparation method of choice for STEM imaging is often FIB

or DualBeam. The preparation process and characterization steps may be

combined very elegantly and conveniently in a DualBeam microscope.

We note that for the point of this discussion, STEM refers to SEM-STEM

only, i.e., imaging with 30 keV transmitted or diffracted electrons in an

SEM (or DualBeam). This is clearly different to STEM in a TEM, where

acceleration voltages of hundreds of keV are typically used, and where higher

performance and analytical capabilities are achieved. STEM should not be

considered as a replacement for TEM inspection, but rather (i) as a tool to be

used prior to TEM, (ii) as an aid in TEM specimen preparation and eva-

luation, and (iii) an extension of the conventional SEM and DualBeam. In

particular, it enables SEM operators to capture images with a spatial reso-

lution of less than1 nm.

5.4.1 Preparation of specimens for STEM characterization

STEM specimens are prepared from a bulk material using FIB and DualBeam

techniques that are identical to TEM specimen preparation [7]. Using

DualBeam systems, it is possible to use STEM imaging during the ﬁnal

thinning of the thin foil. This provides a closed-loop feedback system to the

FIB/DualBeam operator about the thickness and quality of the foil while it is

Focused ion beam systems140

produced. It is possible to use FIB thinning with intermittent STEM inspection,

but on some DualBeam systems, these two steps can even be combined in a

simultaneous process, providing real-time (live) transmission imaging during

the actual thinning using SPI mode.

5.4.2 STEM imaging

STEM imaging is based on the principle of using a focused primary beam of

electrons that is scanned across a thin specimen. At each point of the scan,

the electron beam interacts with the specimen material, either passing or

diffracting through it. The transmitted electrons are collected on a solid-state

detector. When energetic electrons strike a solid-state detector (usually made

from silicon), electron–hole pairs are formed which produce an electrical

current proportional to the energy of the entering electron. This current is

then ampliﬁed in an external electronic circuit, and translated into gray levels

to form an image. The transmission and detection process is illustrated

schematically in Figure 5.10.

Collection of the direct beam yields bright ﬁeld images (labeled ‘‘BF’’ in

Figure 5.10). Bright ﬁeld images yield contrast from mass-thickness effects

and diffraction contrast due to crystalline defects in the specimen. The

interaction of electrons with heavy atoms is stronger than with light atoms,

SEM

Thin sample

SS detector

BF DF HA-DF

Figure 5.10 A schematic diagram of the concept of STEM imaging. The

primary electron beam is scanned through a thin sample. Bright ﬁeld (BF),

annular da rk ﬁeld (DF), and high-angle annular dark ﬁeld (HA-DF) signals

can be collected on the respective segments of a solid-state (SS) detector.

Characterization methods using FIB/SEM DualBeam instrumentation 141

so that areas in which heavy atoms are localized appear darker in contrast

than areas with light atoms. In thicker specimen areas, more electrons are

scattered, causing these areas to appear dark. In addition, defects or grains at

different orientations can also scatter the primary beam, causing diffraction

contrast. In principle, FIB prepared STEM specimens have uniform thickness,

so thickness contrast is usually negligible.

For dark ﬁeld imaging, the opposite result is obtained: heavier and/or

thicker areas will yield a brighter contrast, since more scattering of electrons

occurs and are detected. The scattered electrons are deﬂected from their

straight paths, and are collected on an off-axis segment, or annular portion of

the detector, labeled ‘‘DF’’ in Figure 5.10. Since defects can also scatter the

primary beam, dark ﬁeld imaging can enhance features such as crystal

orientation and other crystalline defects.

High-angle dark ﬁeld detection uses mostly the electrons that are scattered

over the largest angles: this detector segment is placed furthest off-axis. Dif-

fracted beams due to crystalline defects are typically scattered at low angles.

High-angle scattered electrons depend on mass or Z-contrast. Figure 5.11

shows an example of BF, DF, and HA-DF STEM images of the same sample

area. The differences in contrast are evident. In particular, the BF image

shows both mass-thickness effects and diffraction effects. The DF image

shows some reversal of mass-thickness contrast effects compared to the BF

image, but also shows diffraction contrast effects. However, the HA-DF

image shows almost entirely Z-contrast effects; i.e., the heavy W plug shows

the brightest contrast in the image, but some diffraction contrast is also noted.

5.4.3 STEM elemental analysis (EDS)

One of the most appealing features of the SEM(DB)-STEM technique is

the improved spatial resolution that it creates for X-ray energy dispersive

(a) (b) (c)

Figure 5.11 (a) BF, (b) DF, and (c) a HA-DF image of the same sample area

using identical primary beam and scan conditions.

Focused ion beam systems142

spectrometry (XEDS). Since the specimens are electron transparent, the beam

spread and interaction volume normally associated with bulk SEM samples

will be greatly reduced. Therefore, signiﬁcantly higher spatial resolution can

be obtained using the STEM detector while performing X-ray analysis.

Typically, a resolution improvement from hundreds of nanometers or even

some micrometers, to about 30 nm can be achieved. It also provides less

background in the spectrum and allows for better separation of peaks as well as

lower count rate mapping conditions. An example of a high-resolution X-ray

map on a semiconductor device is shown in Figure 5.12. A nitride line with

thickness well below 50 nm is clearly resolved in the XEDS map.

5.5 FIB analytical techniques: particle induced X-rays and

secondary ion mass spectroscopy

5.5.1 Focused ion beam induced X-rays (FIBIX)

As previously mentioned, ion–solid interactions yield a complex combination

of particle sputtering and signal generation, among which is the ejection of

characteristic X-rays. This physical phenomenon is the basis for particle

induced X-ray emission (PIXE). PIXE has generally been performed using

light ions or protons accelerated at energies in the MeV range. To distinguish

that FIB induced X-ray analysis is performed with heavy ions of moderate

energy (i.e., 30 keV), we shall use the term FIBIX. FIBIX can be used in a

DualBeam as a complement to SEM based XEDS analysis [8].

The advantages of ion beam induced X-ray emission are twofold. First, the

FIB induced X-rays originate from a region close to the surface of the sample

material – typically not more than a few tens of nanometers deep (e.g., a

(a) (b)

Figure 5.12 (a) DF STEM image and (b) XEDS map of a semiconductor

device showing high-resolution elemental analysis. The arrows indicate the

presence of the respect ive N, O, and W.

Characterization methods using FIB/SEM DualBeam instrumentation 143

smaller interaction volume compared to SEM). Thus, surface analysis or

accurate depth proﬁling could be achieved. Second, the Bremmstralung is

theoretically orders of magnitude lower. Therefore, ion induced spectra may

provide superior signal to noise ratios over electron beam induced XEDS. A

disadvantage to FIBIX is that a well-deﬁned cut-off energy exists, above

which it is impossible to obtain characteristic X-rays from the target. How-

ever, the technique is most suited for the collection of soft X-rays (< 2 keV),

and is therefore sensitive to light element analysis. As such, FIBIX forms a

good complement to electron induced EDS.

5.5.2 Secondary ion mass spectroscopy (SIMS)

The fundamental process of FIB sputtering is the basis of SIMS. While most

of the sputtered species are neutrals, secondary ions are also emitted. These

secondary ions are detected in an ion mass analyzer such as a quadrupole,

magnetic sector, or time-of-ﬂight (TOF). Correlating the analyzer signal with

the scanning of the ion beam results in ion images with lateral element dis-

tribution. Taking the sputtering yield into account, depth proﬁles can be

obtained as well. Since FIB process mix a small cascade interaction with a

small probe size, lateral resolution of elemental speciﬁc ion images as low as

20 nm may be obtained, while local concentrations of the order of a few

percent can be detected. In comparison, lateral resolution for non-FIB SIMS

ranges from 50 to 200 nm, depending on the primary beam element used.

ion beams offer a very small probe size and are most widely used in

commercial FIB systems. However, they offer a relatively low yield of sec-

ondary ion production. The secondary ion yield for Ga

is typically about a

factor of 40 less than O

, which is used in some non-FIB SIMS equipment [7].

Although superior lateral resolution can be obtained with commercial FIB

instruments, the relatively low secondary ion yield combined with low collec-

tion efﬁciency in quadrupole mass spectrometers limits the utility of elemental

mapping with FIB-SIMS. Enhancement of the secondary ion yield or detection

efﬁciency is needed to increase the sensitivity of FIB-SIMS.

5.6 Other opportunities for in-situ characterization

Focused ion beams, and DualBeams in particular, enable further opportunities

for in-situ characterization. Like SEM, FIB enables the use of mechanical

micromanipulators and electrical probes, for characterization of mechanical

and electrical properties, inside the microscope. Furthermore, it is also possible

Focused ion beam systems144

to combine in-situ fabrication or modiﬁcation of mechanical or electrical

structures with live monitoring of the relevant parameters.

Acknowledgements

Thanks to Hans Mulders and Mark Wall, FEI, for added contributions.

References

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(2003), 1004–15.

[3] S. Reyntjens. Dualbeam (FIB/SEM) sample preparation for sub-micrometer

EBSD analysis. Presented at Advances in Focused Ion Beam Microscopy: FIB

2003, Cambridge, UK (2003).

[4] M. K. Miller, K. F. Russell and G. B. Thompson. Ultramicroscopy, 102 (2005),

287–98.

[5] J. J. L. Mulders and A. P. Day. Three-dimensional texture analysis. Proc.

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[6] J. J. L. Mulders and H. L. Frazer. Microsc. Microanal., 11, Suppl. 2 (2005), 506–7.

[7] L. A. Giannuzzi, F. A. Stevie, eds., Introduction to Focused Ion Beams:

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[8] L. A. Giannuzzi. Scanning, 27 (2006), 165–9.

Characterization methods using FIB/SEM DualBeam instrumentation 145

High-density FIB-SEM 3D nanotomography:

with applications of real-time imaging

during FIB milling

e. l. principe

Carl Zeiss SMT Inc., Redwood City, CA

6.1 Introduction

The ability to acquire, display, and interrogate multi-dimensional volumetric

data sets has been well established through various scientiﬁc disciplines. The

medical ﬁeld in particular has exposed the public to tomographic methods

through now common medical procedures such as computed axial tomo-

graphy (CAT), magnetic resonance imaging (MRI), and positron emission

tomography (PET). In an analogous fashion the focused ion beam (FIB) and

scanning electron microscope (SEM) can combine to generate tomographic

data using the FIB-SEM platform.

Early work completed by a small number of researchers gives an indication

of the variety of information that can be obtained by FIB based tomo-

graphy: Uchick et al. [1] and Kotula et al. [2] have produced secondary

electron (SE) and X-ray images (XEDS) of 3D structures; Dunn et al. [3]

have created 3D mass spectral images using planar FIB etching (FIB-SIMS);

Inkson et al. [4] and Sakamoto et al. [5] have published 3D reconstructions

based on secondary electron images, and Principe [6] has demonstrated the

application of FIB-Auger spectroscopic 3D reconstruction at 10 nm resolu-

tion. Konrad et al. [7] have demonstrated sequential automated acquisition

of 3D electron backscatter pattern (EBSP) tomographic data. This prior

work has shown that FIB based tomographic methods have volumetric data

resolution down to 20 nm or less (i.e., FIB-SE, FIB-Auger) and have tre-

mendous potential for a variety of investigations in both material science and

biology.

Hardware has existed for over a decade to allow collection of volumetric data

from a set of sequential FIB serial sections yet FIB based nanotomography

Focused Ion Beam Systems: Basics and Applications, ed. N. Yao.

Published by Cambridge University Press. ª Cambridge University Press 2007.

146

has until now remained impractical for a routine user. Wider utilization of

FIB based tomographic methods required improvements in the ease of

acquisition, data collection speed, and density of raw data collection.

Advanced FIB-SEM systems such as the CrossBeam family now permit

unattended automated acquisition of volumetric data sets combining XEDS,

EBSP, SE, and backscatter electron (BSE) data. Another enabling factor is

integration of software tools that allow reconstruction, visualization, and

analysis of the data sets including transparency mapping, surface meshing,

exploration of sub-volumes, and animation. This recent evolution in both

hardware and software on FIB-SEM platforms coupled to advanced data

analysis is driving the rapid growth in the ﬁeld of FIB based tomography by

providing accessibility to the routine user.

With the advent of high-resolution simultaneous secondary electron (SE)

and backscatter electron (BSE) imaging during the FIB sectioning process

on the CrossBeam platform it has become practical to acquire several

hundred data frames in the span of less than one hour in a continuous and

automated fashion for volumes of 500 mm

or greater. The approach of

using simultaneous and continuous SEM imaging during the FIB milling

process to create a 3D reconstruction and the method of 3D quantiﬁcation

described in this chapter was developed and ﬁrst demonstrated by the

author. Real-time imaging coupled with automated image recording facil-

itates the data acquisition process while providing volumetric resolution at

the nanoscale. High throughput is also a direct result of being able to

capture high quality image data at high density in real-time during the

milling process.

As will be described through the examples presented, the information

content is not limited to powerful visualization and failure analysis; it is a

research tool that can be applied to quantitatively correlate microstructure,

material properties, and system performance. Aspects of data acquisition,

data processing, and quantiﬁcation will be described.

6.2 Overview of 3D tomography

The word tomography comes from the Greek words tomos (‘‘to cut or sec-

tion’’) and graphein (‘‘to write’’). FIB-SEM nanotomography ﬁts this literal

deﬁnition quite well. In this process the sample is sectioned with the FIB and

the result is ‘‘written’’ in the form of digital data. A three-dimensional

reconstruction results when the individual tomograms are arranged in proper

order to reconstruct representation of the original object. The terminology

for a three dimensional digital image element is known as a voxel, which is

High-density FIB-SEM 3D nanotomography 147